The present invention relates to a wire end or forming section of a paper making machine, particularly a twin wire forming section, and more particularly to the application of pressure to the two wires of a twin-wire forming section for aiding fiber suspension quality.
The invention is developed from the twin-wire former disclosed in International Application WO 91/02842. Two endless forming wire belts form a twin-wire zone, which can be subdivided into three sections.
In the first section, the two wires run over a curved supporting surface of a supporting element, which is there preferably a forming cylinder. They there form a wedge shaped entry nip, to which the headbox directly transmits pulp suspension. In the region of the forming cylinder, some of the water in the fiber suspension is removed downward. Some of the water also penetrates upward through the top wire, on account of the tension of the top wire, and that water is removed by means of subatmospheric pressure. The dewatering pressure is in this case the same in the area where the wire belts wrap around the forming cylinder.
In the second section, there are a plurality of compliantly supported strips which bear against the bottom wire. Between each pair of support strips along the bottom wire, there is a respective fixedly supported strip bearing against the top wire.
The compliantly supported strips in the second section direct forces onto the inner surface of the bottom wire, i.e. the surface of the wire inside its endless loop form. The strips produce linear loads, which induce tolerances in the fiber suspension between the two wires on account of the minimal cross-sectional changes when the suspension flows through at a high operating rate. This avoids flocculation.
In the third section, both wire belts run over a further curved supporting surface of a support element. The support element is preferably in the form of a forming shoe having a curved surface.
The combination of already known features in this three section arrangement ensures relatively good paper web quality with respect to two sidedness, look through and uniformity of formation. However, quality requirements in these respects have increased, so that further improvements are desirable. It is intended that the improvements should also be achievable by means of a simpler configuration of supporting elements. A disadvantage of the above known arrangement is caused by the relative movement between the top wire and the rigid strips. The inner surface of the top wire, inside its loop form, is subject to the effects of wear. Also, due to the in line arrangement of the various supporting elements in the individual sections I, II and III, the wire end of the machine has considerable overall length.
EP 0 516 601 A1 publication discloses pressure elements in the form of flexibly designed blades that may be effective on the wire belts in the region of the individual supporting elements for intensifying the dewatering of the suspension, for accomplishing optimum basis weight distribution of the suspension and also for counteracting flocculation in the fiber suspension. The blades are arranged in such a way that their blade bodies lie substantially transverse to the web running direction and bear with part of their surfaces against one of the wire belts and press that belt against the supporting element. In the configurations described, the blades are effective only as pressure elements, not as water skimming elements.
The support elements take various forms, for example, a wire frame with a closed surface or a surface provided with an opening for the purpose of suction intake, or a forming cylinder or an element designed in the form of a strip.
The arrangement of the flexibly configured blades with respect to the wire belt supported by the supporting surface and also the flexibility of the blades are of significance for the magnitude of the pressure impulses which are introduced. In the configurations described, the magnitude of the pressure impulses is changed mechanically by spindles or pneumatically by hoses which bear against the blade body. By changing the cross section, these produce increased bending stress on the blade bodies and consequently produce an increase in the contact pressure of the part of the blade body bearing against the wire belt. The blade arrangement with the associated adjusting mechanism for changing the contact pressures is a relatively complicated structure. Thus, substantially only one installation position of the blades is possible. The magnitude of the contact pressures is dependent on the properties of the blades, and specifically on the flexibility of the blade material. In cases of low contact pressures, high blade flexibility and clotting of the fiber suspension, there is the risk of the blade body oscillating. The contact pressure and consequently the hydraulic pressure in the suspension or the pressure impulse effective in the suspension are also dependent on the blade angle. The effects of wear at the blade cutting edge result, however, in changing of the blade angle. This is found to be a particular problem if there is uneven wearing of the blade cutting edge over the width of the machine.
A further disadvantage of this configuration is that clots, which form from fine fibers which penetrate through the wire mesh during dewatering and which can form upstream and downstream of the blade, enter into a wedge which is formed, by the arrangement of the blade and the supporting surface, between the blade and the supporting surface and damage the wires there.